Did you download all the BIOS and other drivers for his PC? You might need a specific one for that adapter as well.
I assume this pc is pretty new, but maybe his network adapter settings also got messed with. I’d cross reference his and your settings to make sure they match up, and if that doesn’t help he may have a network card or motherboard issue
I’d also try removing the killer adapter to see if maybe that’s what is causing the issue
His computer is my old computer, all of the software is the same from 8 years ago. I did reformat it before handing it over but it was stable a few days ago when he was playing d2, we played for a few hours, and now suddenly it doesn't want to stay connected.
The killer adapter is integrated into the motherboard, it's either I use that or I use WiFi or a card. I did buy a card as a backup but I'd love to get this hardwired. No one wants a wireless PC.